import java.util.Arrays;

// 归并排序
public class MergeSort {
    public static void process(int[] arr, int L, int R) {
        if (L == R) {
            return;
        }
        int mid = L + ((R - L) >> 1);
        process(arr, L, mid);
        process(arr, mid + 1, R);
        merge(arr, L, mid, R);
    }

    public static void merge(int[] arr, int L, int M, int R) {
        int[] help = new int[R - L + 1];
        int i = 0;
        int p1 = L;
        int p2 = M + 1;
        while (p1 <= M && p2 <= R) {
            help[i++] = arr[p1] <= arr[p2] ? arr[p1++] : arr[p2++];
        }
        while (p1 <= M) {
            help[i++] = arr[p1++];
        }
        while (p2 <= R) {
            help[i++] = arr[p2++];
        }
        for (i = 0; i < help.length; i++) {
            arr[L + i] = help[i];
        }
    }

    public static void main(String[] args) {
        int[] arr = {1,4,2};
        process(arr, 0, arr.length-1);
        System.out.println(Arrays.toString(arr));
    }
}

// 小和问题
/**
 * InnerMergeSort
 */
class InnerMergeSort {
    public static int process(int[] arr, int l, int r) {
        if (l == r) {
            return 0;
        }
        int mid = l + ((r - l) >> 1);
        return process(arr, l, mid) + process(arr, mid+1, r) + merge(arr, l, mid, r);
        
    }

    public static int merge(int[] arr, int l, int m, int r) {
        int [] help = new int[r-l+1];
        int i = 0;
        int p1 = l;
        int p2 = m + 1;
        int res = 0;
        while (p1 <=m && p2 <= r) {
            res += arr[p1] < arr[p2] ? (r - p2 + 1) * arr[p1] : 0;
            help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
        }
        while (p1 <= m) {
            help[i++] = arr[p1++];
        }
        while (p2 <= r) {
            help[i++] = arr[p2++];
        }
        for (i = 0; i < help.length; i++) {
            arr[l+i] = help[i];
        }
        return res;
    }
 
    public static void main(String[] args) {
        int[] arr = {1,4,2};
        System.out.println(process(arr, 0, arr.length-1));
    }
}


// 逆序对问题